That's the only thread active, making it likely that we encountered a TIMEOUT,
and the TIMEOUT handler killed all childs.

We lose() with "Trapping to run pending handler while GC in progress."
in interrupt_hander_pending; *GC-PENDING* is :IN-PROGRESS at that
point.

Because *GC-PENDING* is :IN-PROGRESS, and the only place it's
set to that is SUB-GC -- and shortly later in SUB-GC, it's set to NIL,
it seems like there was a transfer-of-control out of SUB-GC before
that setting.

And, indeed, some clever trick involving *BREAK-ON-SIGNALS*
revealed:

debugger invoked on a SIMPLE-CONDITION in thread #<THREAD "initial thread" RUNNING {100382A091}>:
The value -4 is not of type (MOD 1152921504606846973).
BREAK was entered because of *BREAK-ON-SIGNALS* (now rebound to NIL).

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
0: [CONTINUE] Return from BREAK.
1: [REASSIGN] Return from BREAK and assign a new value to *BREAK-ON-SIGNALS*.
2: [ABORT ] Exit debugger, returning to top level.